the mission of the nevada agricultural experiment …naes.agnt.unr.edu/pms/pubs/50_2009_01.pdfthe...

The mission of the Nevada Agricultural Experiment Station is to develop

research capacity to support economic development in the state of Nevada,

especially in the agricultural sector, and to disseminate new research-

based information to stakeholders. In support of this mission, the Nevada

Agricultural Experiment Station offers outreach research publications for

use by agricultural producers throughout the state. This NAES Research

Bulletin provides peer-reviewed research findings that are applicable to

Nevada agriculture and based on excellent science. We trust you will find

this bulletin informative and useful.

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Wyoming Big Sagebrush

State & Transition Model

And Management Key for Nevada

First Approximation

Barry Perryman1, Sherman Swanson2, Gary Brackley3,

Brad Schultz4, Paul Blackburn5, Kent McAdoo6, Gary Medlyn7 and Robin Tausch8

INTRODUCTION

During the late twentieth century, public perception of Wyoming big

sagebrush (Artemisia tridentata ssp. wyomingensis) [Beetle & A. Young]

changed from being a ‘weed’ to a valuable resource in danger of extirpation in

some landscapes. The two polar perspectives, perceiving it to be of no value

and only competitive with grasses, or perceiving it to be so valuable and scarce

that we must never control it, neither serve or benefit land managers or the

wildlife that depend on this important habitat. During this period, a focus on

rangeland condition has shifted to a focus on ecological thresholds and the

information needed to allocate limited financial and other resources to those

1 Rangeland Ecologist, Nevada Agricultural Experiment Station and College of Agriculture, Biotechnology and Natural Resources Department of Animal Biotechnology, Reno NV 89557 2 State Range Specialist, University of Nevada Cooperative Extension and College of Agriculture, Biotechnology, and Natural Resources Department of Natural Resources and Environmental Science, 1000 Valley Rd. Reno NV 89512 3 State Range Conservationist, Natural Resources Conservation Service Nevada State Office, 1501 Longley Ln, Reno NV 89502 4 Humboldt County Educator, University of Nevada Cooperative Extension, 1085 Fairgrounds Rd., Winnemucca NV 89445 5 Soil Scientist, Natural Resources Conservation Service Elko Field Office, Elko, NV 89801 6 Northeast Nevada Natural Resources Specialist, University of Nevada Cooperative Extension, 1500 College Parkway, Elko, NV 89801 7 Soil Scientist, Bureau of Land Management Ely District, Ely NV 89315 8 Research Scientist, USDA Forest Service

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areas, times, and actions that are most important for maintaining rangeland

health.

This publication focuses on ecological sites capable of supporting plant

communities dominated by Wyoming big sagebrush. This shrub occurs at lower

elevations on valley bottoms, alluvial slopes, foothills, and mountain side slopes.

It typically inhabits areas too moist for salt desert shrub species and too dry for

mountain big sagebrush. While this subspecies is somewhat palatable to sheep

and mule deer, it is not as palatable as black sagebrush (Artemisia nova

[Nelson]) nor is it palatable to cattle.

This state and transition model and management key describes vegetation

change and management alternatives for the Natural Resources Conservation

Service ecological sites listed in Table 1. However, some areas where these

sites occur are better understood by disregarding their potential for transitioning

to a tree state because they are logistically far from those sites that generally

have juniper (Juniperus sp.) and/or pinyon pine (Pinus monophylla Torr. & Frem.)

trees. In general, the potential for transitioning to the tree state is greater for

Wyoming big sagebrush sites that have higher precipitation or elevation, have

deeper soils, or are closer to sites with these characteristics. Similarly, the

potential for transitioning to the tree state is greater for Wyoming big sagebrush

areas that have closer proximity to trees.

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Table 1. Ecological sites described by this model in Nevada (NRCS Ecological

Site Descriptions, 2003).

Site number Ecological site name Site number Ecological site name 023XY011NV Dunes 8-10" P.Z. 026XY099NV Coarse Loamy 8-10" P.Z. 023XY020NV Loamy 10-12" P.Z. 026XY100NV Stony Slope 10-12" P.Z. 023XY030NV South Slope 8-12" P.Z. 026XY102NV Gravelly Clay Loam 8-10" P.Z.023XY033NV Clayey 10-14" P.Z. 027XY007NV Loamy Slope 8-10" P.Z. 023XY038NV Droughty Loam 8-10" P.Z. 027XY008NV Droughty Loam 8-10" P.Z. 023XY039NV Loamy Slope 10-14" P.Z. 027XY029NV Gravelly Fan 8-10" P.Z.

Table 1 Continued Site number Ecological site name Site number Ecological site name

023XY040NV Granitic Fan 8-10" P.Z. 027XY045NV Sandy 8-10" P.Z. 023XY049NV Granitic South Slope 8-12" P.Z. 027XY051NV South Slope 8-10" P.Z. 023XY051NV Sandy 8-12" P.Z. 027XY054NV Loamy Slope 10-12" P.Z. 023XY057NV Granitic Loam 10-12" P.Z. 027XY058NV Loamy 10-12" P.Z. 023XY063NV Shallow Granitic Hill 10-14" P.Z. 027XY065NV Granitic Slope 8-10" P.Z. 023XY068NV Granitic Loam 8-10" P.Z. 027XY067NV Granitic Loam 8-10" P.Z. 023XY071NV Ashy Loam 10-12" P.Z. 027XY072NV Granitic Slope 10-12" P.Z. 023XY072NV Ashy Slope 10-12" P.Z. 027XY088NV Granitic Loam 10-12” P.Z. 023XY077NV Shallow Loam 10-14" P.Z. 027XY091NV Loamy Fan 10-12” P.Z. 023XY082NV Loamy Fan 10-12" P.Z. 027XY092NV Granitic Fan 10-12” P.Z. 023XY088NV Chalky Knoll 028AY005NV Sandy 8-10" P.Z. 023XY096NV Ashy Sandy Loam 10-12" P.Z. 028AY010NV Coarse Gravelly Loam 10-12" P.Z.023XY097NV Loamy Fan 8-10" P.Z. 028AY015NV Loamy 8-10" P.Z. 023XY099NV Channery Hill 8-10" P.Z. 028AY017NV Shallow Loam 8-10" P.Z. 023XY101NV Stony Slope 8-10" P.Z. 028AY022NV Gravelly Clay 8-10" P.Z. 023XY102NV Gravelly Clay Slope 10-12" P.Z. 028AY028NV Droughty Loam 8-10" P.Z. 024XY001NV Dunes 6-10" P.Z. 028AY031NV Loamy Fan 8-10" P.Z. 024XY005NV Loamy 8-10" P.Z. 028AY040NV Gravelly Loam 10-12" P.Z. 024XY006NV Dry Floodplain 028AY050NV Gravelly Clay 10-12" P.Z. 024XY013NV Loamy 10-12" P.Z. 028AY054NV Coarse Loamy Fan 8-10" P.Z.024XY017NV Sandy 8-10" P.Z. 028AY086NV Coarse Loamy Fan 10-12" P.Z.024XY020NV Droughty Loam 8-10" P.Z. 028AY091NV Loamy Fan 10-14" P.Z. 024XY026NV Stony Slope 6-10" P.Z. 028AY095NV Loamy 10-12" P.Z. 024XY028NV South Slope 8-12" P.Z. 028AY121NV Deep Loamy 8-10" P.Z. 024XY033NV Steep North Slope 10-12" P.Z. 028AY124NV Loamy Plain 024XY035NV Shallow Loam 10-14" P.Z. 028BY005NV Sandy 8-10" P.Z. 024XY045NV Eroded Slope 6-10" P.Z. 028BY007NV Loamy 10-12" P.Z. 024XY046NV Gravelly North Slope 028BY010NV Loamy 8-10" P.Z. 024XY047NV Shallow Loam 8-10" P.Z. 028BY014NV Loamy Plain 8-10" P.Z. 024XY058NV Sandy Loam 8-10" P.Z. 028BY045NV Loamy Fan 8-12" P.Z. 025XY013NV Churning Clay 8-12" P.Z. 028BY052NV Droughty Loam 8-10" P.Z. 025XY014NV Loamy 10-12" P.Z. 028BY054NV Silty Plain 8-10" P.Z. 025XY015NV South Slope 8-12" P.Z. 028BY056NV Silt Flat025XY019NV Loamy 8-10" P.Z. 028BY068NV Dune 8-10" P.Z. 025XY021NV Shallow Loam 8-12" P.Z. 028BY080NV Shallow Loam 8-10" P.Z. 025XY027NV Loamy 12-14" P.Z. 028BY082NV Loamy Fan 12+" P.Z. 025XY045NV Ashy Loam 8-10" P.Z. 028BY086NV Gravelly Clay 10-12" P.Z. 025XY066NV Ashy Loam 10-12" P.Z. 028BY094NV Calcareous Loam 10-14" P.Z.025XY070NV Loamy Fan 8-10" P.Z. 029XY006NV Loamy 8-10" P.Z. 026XY010NV Loamy 10-12" P.Z. 029XY010NV Loamy Slope 8-10" P.Z. 026XY011NV South Slope 8-10" P.Z. 029XY029NV Loamy 10-12" P.Z. 026XY015NV Shallow Loam 10-12" P.Z. 029XY049NV Sandy Loam 8-12" P.Z. 026XY016NV Loamy 8-10" P.Z. 029XY057NV Loamy Slope 12-14" P.Z. 026XY019NV Churning Clay 10-12" P.Z. 029XY073NV Bouldery Loam 8-12" P.Z. 026XY020NV Sandy 8-10" P.Z. 029XY075NV Loamy Slope 10-12" P.Z. 026XY022NV Stony Slope 8-10" P.Z. 029XY105NV Gravelly Clay 10-12" P.Z. 026XY024NV Droughty Loam 8-10" P.Z. 029XY106NV Gravelly Clay Slope 10-12" P.Z.026XY026NV Granitic Slope 10-12" P.Z. 029XY114NV Loamy Fan 8-10" P.Z. 026XY029NV Eroded Slope 8-12" P.Z. 029XY116NV Loamy Plain 026XY051NV Dune 8-10" P.Z. 029XY117NV Silty Plain026XY096NV Sandy Plain 029XY119NV Silt Flat026XY098NV Gravelly Loam 8-10" P.Z. 029XY158NV Coarse Loamy 8-10" P.Z.

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For the areas where this model and management key applies, we discuss two

management situations: 1) areas where cheatgrass (Bromus tectorum L.) and

other invasive weeds (annuals and perennials) are established and becoming, or

are already an important management factor; and 2) areas where natives are the

only ecologically important species established in the area or at least they still

dominate ecological processes and management concerns.

1) MANAGEMENT WITH CHEATGRASS AND OTHER INVASIVE WEEDS

On these landscapes, the presence of exotic and/or noxious, invasive weeds

threatens the natural resilience and utility of most if not all Wyoming sagebrush

plant communities. These species compete very effectively with native plants.

They can transition plant communities to new states (Fig.1) or dominate after

certain disturbances without appropriate and timely management action. Their

presence is always a hazardous situation. When common, their presence

typically results in a transition to a new state because the exotic weeds, not the

desired species, determine ecological processes.

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sorbitol positive

Introduction

PERENNIAL HERBACEOUS STATE

DIMINISHED PERENNIAL HERBACEOUS UNDERSTORY

PERENNIAL HERBACEOUS UNDERSTORY

MOSTLY ABSENT

REQUIRES ADDITIONAL INPUTS.

SHRUB STATE

PJ CONTROL AND NEW UNDERSTORY PROPAGULES.

DIMINISHED SHRUB S

TREE STATE

?

ALTERED SITE POTENTIAL STATE

WYOMING BIG SAGEBRUSH STATE & TRANSITION MODEL (WITH CHEATGRASS AND/OR OTHER INVASIVE WEEDS ON SITES CAPABLE OF SUPPORTING TREES)

SHRUB AND TREE

REMOVAL AND

SUCCESSFUL

SEEDING

SHRUB AND TREE

REMOVAL

ANNUAL GRASS FIRE CYCLE STATE

FIRE RESISTANT SHRUBS AND CHEATGRASS

CHEATGRASS AND OTHER ANNUALS

ACCELERATED

EROSION

FIRE OR PJ CONTROL WITHOUT

SUCCESSFUL SEEDING

?

PERENNIAL INVASIVE WEED STATE

EXPLOSION OF

PERENNIAL INVASIVE

SPECIES

SEEDED PERENNIAL HERBACEOUS STATE

FIRE OR FIRE SURROGATE

SUCCESSION

SEEDED HERBACEOUS WITH SAGEBRUSH

FIRE O

RP

jC

ON

T RO

L AN

D

S UC

CE S SF U

L SE E D

I NG

SUCCESSION

FREQUENT FIRESINTRODUCTION AND EXPLOSION OF

PERENNIAL INVASIVE SPECIES

REL

ATI

VE

AB

UN

DA

NC

E

EARLY SERAL DOMINANTS

HERBACEOUS PERENNIALS SAGEBRUSH OVERSTORY

DOMINANT TREE SPECIES

SAGEBRUSH UNDERSTORY

FIRE A

ND

WEED

EXPLO

SION

EXPLOSION OF

PERENNIAL

INVASIVE SPECIES

SUCCESSION


SEEDED HERBACEOUS

NO

FIRE

UNDERSTORY MOSTLY ABSENTHERBACEOUS

PLANT COMMUNITY

SHRUBBY OVERSTORYWITH HERBACEOUS

UNDERSTORY

NO FIRE TOBALANCE GRAZING

ANNUAL GRASS FIRE CYCLE STATE AFTER FIRE AND WITHOUT SUCCESSFUL SEEDING

PERENNIAL INVASIVE WEEDS DOMINATE IF THEIR POPULATIONS ARE ALLOWED TO EXPLODE

NO FIRE

NO

FIRE

FACILITATED SUCCESSION

NO FIRE

Fig. 1. Each box is shown as a different color to identify that it is a different state. The solid-line arrows between boxes are irreversible transitions across thresholds. Dashed-line arrows show active restoration of ecological processes. Inset box shows relative abundance of plant groups and relative sequence of transitions through succession without proactive management.

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PERENNIAL HERBACEOUS STATE Description: The plant community is dominated by deep-rooted perennial

bunchgrasses, with perennial forbs and varying amounts of Wyoming big

sagebrush and other shrubs. Sagebrush can dominate the plant community and

juniper and/or pinyon pine trees may be present as seedlings, saplings, or sparse

mature trees as long as the understory remains robust. If the perennial

understory is dense and vigorous enough to recover quickly after being released

from the competition of woody plants, the vegetation has not crossed a threshold

to the shrub or tree state. Descriptions of the ecological sites listed in Table 1

provide relative species composition and production data for each ecological site

in this perennial herbaceous state. Cheatgrass (or other non-native annual

plants) is a minor component of the understory vegetation.

Successional trajectories: The perennial herbaceous state plant community is

resilient or cyclic because secondary succession processes and disturbance

regimes are functional. Periodic release of understory perennials from increasing

competition from sagebrush is facilitated primarily by fire. However, other causes

for widespread shrub die offs have been noted. Normal fire frequency is

approximately 50-100 years (Wright and Bailey 1982). Without woody plant

removal, the plant community transitions to the shrub state or if trees are present,

to the tree state. On drier sites juniper may increase and on more mesic sites,

pinyon may increase. As transition to shrubs or trees occurs, the proportion of

cheatgrass in the herbaceous understory increases while perennial herbaceous

species decline. Poor grazing management of large domestic and/or wild

herbivores can diminish the vigor and expression of palatable perennial

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herbaceous plants. Removal of deep-rooted species may leave only or primarily

Sandberg bluegrass (Poa secunda [J.S. Presl.]) or cheatgrass. This makes

summer moisture and other resources more available to non-palatable shrubs

and/or trees and accelerates and increases the likelihood of transition to the

shrub, tree, or annual grass fire cycle state.

Management strategies to maintain the state: Manage for vigor, density, and

diversity of perennial herbaceous species. Include sagebrush and other woody

species in management objectives as desired. However, manage for no more

shrub and young tree canopy cover than is appropriate for the site in order to

maintain a resilient understory. Management should not allow the plant

community to transition across a threshold to the shrub or tree state. Research

indicates that herbaceous biomass production begins to decline when sagebrush

cover reaches 5-7% and density begins to decline when sagebrush cover

reaches about 12-15%, depending upon site potential (Rasmussen and others

2001, Winward 1991). To increase the vigor and density of the native perennial

herbaceous plants, intervene with mechanical control measures, prescription

grazing, herbicides, or very judicious use of prescribed fire. Shrub and young

tree control should be practiced as woody plant cover increases toward threshold

levels. However, caution is advised because cheatgrass can erupt from a seed

bank soon after control of woody plants opens niches that a sparse understory

cannot rapidly fill. The winter-annual, cheatgrass, out competes perennial

seedlings in most years on all but the sandiest soils. To minimize die off of

perennial herbaceous plants and bare patches, woody plant management may

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be needed more frequently than where only native perennial plants occur in the

understory. Where soils are erodible, minimize soil surface disturbance.

Wherever treatments disturb soil, ensure that adapted perennial plants or seeds

are available to compete with cheatgrass given the specific treatment conditions,

such as seedbed preparation, grazing regime, etc.

Grazing management should be designed to foster perennial herbaceous

species in the community. Excessive, prolonged, or poorly timed grazing by

animals with herbaceous diet preferences can increase shrubs. Shrubs can be

decreased by relatively intense winter grazing by animals with shrub diet

preferences. Supplemental feeding to concentrate cattle for mechanical damage

controls sagebrush in small patches, especially when shrubs are dry or frozen

and brittle. To limit bare ground after future disturbances, grazing and other land

or vegetation management actions should not weaken the perennial herbaceous

community. Where perennial herbaceous understory is weak and shrub cover is

still well below maximum, consider using selective herbicides to manage

cheatgrass and/or adjusting grazing management to restore vigor and density of

desirable understory species several growing seasons prior to controlling shrubs.

Bare ground is more susceptible to accelerated erosion, and invasive plants

establish faster in open niches. Management to maintain the perennial

herbaceous state (prescribed grazing and periodic control of woody plants) is

much more cost effective than management to return to this state once a

threshold has been crossed (control of woody plants, weed control, reseeding,

and temporary rest from grazing).

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SHRUB STATE

Description: Shrub cover has increased and perennial herbaceous understory

cover has decreased across a threshold level. Deep-rooted, perennial

bunchgrasses are rare to absent in the understory. The cheatgrass component

varies from present to dominant in the herbaceous understory. This state is very

susceptible to invasion by annual weeds before and especially after fire or other

large scale disturbance. Wyoming big sagebrush and other shrubs dominate the

plant community. Juniper and/or pinyon pine trees may be established on the

site but do not yet dominate ecological processes.

Successional trajectories: Native herbaceous understory is diminished from

perennial herbaceous state levels and may be absent or nearly so when

sagebrush cover reaches its maximum for the site. The relative abundance of

cheatgrass in the understory increases as perennial grasses decline. Eventually

cheatgrass dominates the sparse understory but drives long-term community

change for both shrubs and herbaceous species. Because a threshold has been

crossed, removal of grazing pressure will not restore the native herbaceous

component. This will coincidently require fire or other shrub control measures.

However, burning or other woody plant control measures without reseeding will

not return a mix of deep-rooted bunchgrasses and other plants characteristic of

the perennial herbaceous state. Return to the perennial herbaceous state

requires shrub control, cheatgrass control, reseeding, and possibly additional

management, depending on site-specific conditions. Return to the perennial

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herbaceous state requires facilitated succession beginning with transition to the

seeded perennial herbaceous state.

Fire or other major disturbance will increase the abundance of cheatgrass

and other annuals. A decline of big sagebrush in the overstory, coupled with an

increase in cheatgrass density indicates a transitional pathway to the annual

grass fire cycle state. With increasing cheatgrass fuel loads, the threat of wildfire

increases due to better fuel continuity and much higher flammability of this fine-

stemmed, often evenly distributed, early-growing and early-drying annual grass.

This change in fuel characteristics indicates a transition to the annual grass fire

cycle state that is completed by an inevitable fire. If trees are present, fire is

delayed, and tree invasion is not controlled, the plant community will transition to

the tree state. At the landscape scale, the rate of transition largely depends on

the geographic extent of fires, and can be quite extensive (up to 100,000 ac or

more).

Management strategies: To maintain the shrub state, or at least sagebrush,

prevention of wildfire is critical. Strategies often include creation of green strips

or other fuel breaks to keep wild fires small so that all sagebrush habitats are not

lost at once. Prescribed grazing may be used to reduce fine and/or woody fuels.

To transition to the seeded perennial herbaceous state, apply shrub and

weed control in conjunction with reseeding operations. Shrub control measures

could include herbicide, mechanical, or shrub-consuming herbivore treatments,

the judicious application of prescribed fire or wildfire. After wildfire or other shrub

removal, reseeding becomes urgent. Seeding is absolutely required before or

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within the first fall or early winter after shrub control. Thereafter, competition from

a rapidly expanding cheatgrass population may prevent seeding success.

Reseeding requires cheatgrass control unless a very hot fire removed all but 0-3

cheatgrass seeds per square foot. It also requires appropriate seedbed

preparation, planting date, and follow-up management. Reseeding treatments

could include native perennials, grasses, forbs and shrubs, and/or adapted non-

native perennial species. Where soil stabilization following wildfire is a priority

objective, seeding non-native perennial grasses with high seedling vigor may be

the best option. Investigate the feasibility of facilitated succession, seeding

initially with adapted non-native grasses and later inter-seeding with adapted

native herbaceous and/or shrub species.

SEEDED PERENNIAL HERBACEOUS STATE

Description: The choice of species in the seed mix, species in the pre-existing

seed bank, and the growing conditions in the first few years after the seeding

largely determine the species composition of the seeding. On many Wyoming

big sagebrush sites, few species will do predictably well. Even for crested

wheatgrass (Agropyron cristatum L. Gaertner), the most commonly seeded and

most dependable species, moisture following seedings is sometimes insufficient,

especially on the driest sites with the greatest soil limitations. Because perennial

bunchgrasses provide a clumped fuel composed of coarser stems that stay

green longer than cheatgrass, they generally reduce fire spread rates, and the

fire interval is generally long enough to allow sagebrush to become well

established (unless seeding design or management precludes it). Although

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functionally quite similar to the perennial herbaceous state, the seeded perennial

herbaceous state is shown as a separate state because there is always an initial

loss of genetic diversity once seeding is required.

Successional trajectories: Seedings often begin with expression of early seral

species present in the seed bank such as annual forbs. As perennials and

shrubs become better established, they typically exclude or severely diminish all

but the most aggressive of these early seral species or limit them to small

disturbed areas. If sagebrush was initially established by seeding or from seeds

left in safe sites, it structurally dominates a seeding more quickly. If not,

recruitment occurs from the edges or from unburned or untreated shrub islands.

Other species colonization depends on the mechanisms of seed dispersal and

their success in finding favorable microsites, or on treatments designed to

facilitate succession through their establishment. Transition back to the

perennial herbaceous state involves replacing the seeded species with native

species and/or increasing species diversity including the addition of shrubs,

especially sagebrush.

Management strategies: Seeding size, shape, or amount of edge and

orientation with respect to prevailing winds, as well as fire management

strategies to leave shrub islands or create a mosaic, can influence the process of

sagebrush re-establishment. During and after seeding establishment, livestock

grazing can be used to encourage niches for sagebrush and other species that

may be present. Conversely, grazing can be discouraged or managed

conservatively to favor only those species more palatable to livestock. Once the

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seeding has been used to avoid the transition to an annual grass fire cycle state,

management and additional seeding can be used to facilitate succession toward

various species compositions. Natives can be interseeded but often do not

compete well with established species unless steps like soil disturbance are used

to open new niches. Often the focus for management is simply maintaining the

seeding. This requires maintenance of ecological processes. Appropriate

activities would include keeping the seeding from becoming so dominated by

shrubs or weeds that the seeded understory is unable to survive a fire, or

allowing sufficient grazing to maintain plant vigor and reduce wolf plants.

Management strategies described for the perennial herbaceous state also apply

to the seeded perennial herbaceous state. However, specifics of grazing

management may differ according to the needs of the seeded and other desired

species.

TREE STATE

Description: Juniper and/or pinyon pine has established on a site and has

caused a decline in understory (herbaceous and shrub) cover and production

due to extended fire return interval. Although trees generally establish under

shrub canopies, they can invade the perennial herbaceous, seeded perennial

herbaceous, and shrub states. Trees have assumed ecological dominance,

driving future ecological processes. Understory (herbaceous and/or sagebrush)

has decreased across a threshold level defined by its lack of resilience to a tree-

removing disturbance. Tree biomass now dominates the plant community, with

leaf and fuel biomass as much as seven to eleven times the levels of perennial

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herbaceous or shrub states. However, tree cover is highly concentrated, often

leaving large bare interspaces that are susceptible to rill erosion, especially on

drier sites. Cheatgrass is present and often dominates the understory as trees

mature. Although live cheatgrass density and vigor may be lower in the tree

state than in other states, its seed bank is often extensive.

Successional trajectories: Herbaceous and/or shrub understory is diminished

from previous state levels to almost absent where trees are mature and the site

fully stocked. Shrub leaf biomass declines to approximately 20-25% of potential

when tree leaf biomass approaches 50% of maximum potential for the site

(Tausch and West 1995). During this process a 10 % increase in tree cover can

result in a 50% decline in understory production. The degree of resilience of the

understory is determined in part by the tree-removing disturbance. A very hot

wildfire may remove remaining herbaceous species and their seed reserves

(indicating the threshold to the tree state has been crossed) while a more gentle

form of tree removal may release these species from the tree competition

(indicating that the threshold had not yet been crossed). Once one or more

thresholds have been crossed in getting to the tree state, return to the perennial

herbaceous state requires transition to the seeded perennial herbaceous state

first. This requires shrub and/or tree control, reseeding, and usually other

management actions such as weed control. Cheatgrass is present and its

proportion in the understory tends to increase as native understory species

decline. If the native perennial understory is absent or sparse, fire or other tree

control measures alone will not increase most herbaceous/shrub understory

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species. Rather, the cheatgrass seed bank will increase cheatgrass abundance

after the release from tree competition, and transition the site to the annual grass

fire cycle state. Major soil erosion events after fire from severe wind, or from

major precipitation events on moderate or steeper slopes, can trigger a transition

to an altered site potential state. Mature tree stands may increase this risk by

allowing rills to form in large bare interspaces.

Management strategies: To manage this state for continued tree production,

protection from wildfire is essential. However, as trees grow, fuel accumulates

and tree canopies grow closer to each other. This increases the likelihood of a

catastrophic fire spreading across the landscape. Thinning a stand reduces fuel

loads. However, larger bare interspaces increase erosion hazard. Continued net

fuel production on this type often increases the risk of fires in neighboring

woodland types including areas where trees are very old because fire was

historically rare or involved only single trees. Management plans designed to

break up the landscape scale continuity of fuels with firebreaks, greenstrips, or

imposed differences in vegetation structure serve to reduce the risk of large fires

that leave watersheds barren. Applying tree control and rehabilitation treatments

in smaller patches increases the likelihood of fires creating a diverse mosaic of

habitats. This reduces the cost of future fire fighting, increases the opportunity

for fire use, and increases sustainability of ecological processes.

To transition to the seeded perennial herbaceous state, apply tree and

weed control and seed adapted perennials. After successfully attaining the

seeded perennial herbaceous state, facilitated succession can return the site to

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the perennial herbaceous state. Tree control measures could include prescribed

fire, mechanical, prescribed grazing, or herbicide treatments. Restoration

requires the use of site-adapted grass, forb, and shrub species and methods. If

site stabilization is a priority objective, non-native perennial herbaceous species

may be the best option for revegetation. Rehabilitation is required in the fall or

early winter immediately following tree removal.

ANNUAL GRASS FIRE CYCLE STATE

Description: Cheatgrass and/or other annual grasses and forbs (e.g., mustards)

dominate the herbaceous community. Most perennial herbaceous species

cannot compete with the dense population of cheatgrass and are absent or

nearly so. Fire intervals often shorten to 2 to10 years. Sagebrush is generally

unable to survive and reproduce with this fire frequency. Sprouting, fire-tolerant

shrubs may form a shrub overstory where fires are too frequent for sagebrush

but infrequent enough to support sprouting shrubs.

Successional trajectories: This plant community is functionally an annual

grassland. Cheatgrass initially dominates the site following wildfire. Sprouting,

fire-tolerant shrubs are the only woody plants, and these shrubs may eventually

dominate structural aspects of the area if fires are not too frequent. Cheatgrass

and annual forbs become the dominant vegetation after frequent, repeated fires.

Poor grazing management can shift species composition toward less palatable

species decreasing vegetative cover, while increasing erosion hazard. The level

of risk has increased for the community to transition to a perennial invasive weed

state. If perennial invasive species such as knapweeds (Centaurea sp.) are

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introduced, the plant community could rapidly become dominated by these

species, crossing a threshold to the perennial invasive weed state. This makes

rehabilitation to a seeded perennial herbaceous state more difficult, even with

extensive and intensive inputs. Fires export nitrogen and frequent fires may shift

the plant community toward undesired species tolerant of low-nitrogen soils.

Repeated fires expose soil to erosion more often, facilitating transition to the

altered site potential state during severe hydrologic or wind events.

Management strategies: To manage this state for continual annual grass

production, apply proper grazing for annual grassland. Leave sufficient biomass

and time for seed production and soil protection while consuming sufficient fuel to

reduce fire risk. This can be challenging due to highly variable production and

timing of production among wet and dry years. In the wettest years, grazing may

consume abundant forage in only some pastures or use areas or, with sheep

herding, in fuel breaks. In the driest years forage may be essentially absent.

To transition to the perennial herbaceous state, cheatgrass control and

reseeding operations are required. Mechanical, chemical, and grazing

treatments can reduce cheatgrass seed production. If fire intolerant shrubs like

sagebrush are included in the seed mix, a fuel management strategy must be

employed to reduce fire danger to shrub seedlings. Prescription grazing and

green stripping can be used across a landscape to reduce fuel loads, fire size,

and frequency. Establishment of seeded perennial herbaceous species (such as

bunchgrasses) will also reduce fuel continuity, potentially reducing the rate of fire

spread and size.

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PERENNIAL INVASIVE WEED STATE

Description: Although this state has yet to be identified in Nevada, one or more

of the weeds listed on the Nevada noxious/invasive weed list, or a new invasive

weed, could dominate the herbaceous vegetation, competitively excluding native

perennial herbaceous dominants. Weeds may burn readily and typically exclude

sagebrush and/or pinyon and juniper trees. Their competitive advantage, in an

environment without diseases, or insects from their ancestral home allows them

to displace most other plants to form virtual monocultures. Initial weeds may

facilitate the establishment of even more competitive invasive weeds.

Successional trajectories: The risk of transitioning to the perennial invasive

weed state increases after transition to the Shrub, Tree, Annual Grassland Fire

Cycle, and the Altered Site Potential States. Risk increases as soon as invasive

perennial plants, such as one of several knapweed species, begin to colonize an

area unless they are eradicated immediately upon discovery. Otherwise, initial

colonization generally expands toward a monoculture. Experience in other parts

of the western United States demonstrates the highly competitive nature of some

invasive weeds. However, which species will be most competitive on each

ecological site, state, or phase is still unknown. As initial infestations change

species composition and/or soil characteristics and site potential, other weeds

will likely become more competitive, causing instability in species composition.

Many invasive weeds are competitive, but do not effectively protect soil from

erosion or they are highly flammable, leading to unprotected soils after fires,

increasing the risk of crossing a threshold to the altered site potential state.

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Management strategies: Invasive plant colonies should be eradicated

immediately upon discovery. Once invasive weeds dominate a site, the expense

of weed control, follow-up control, and revegetation treatments generally exceed

on-site economic returns. However, these management strategies are justified to

quarantine weeds in one area, reducing spread potentials. Herbicides,

prescribed grazing (goats), and/or hand grubbing should be used to eradicate

small populations. Where eradication is no longer possible, mechanical,

chemical, and/or biological controls such as insects or prescribed grazing should

be used to control or confine infestations. Weed control areas will require

reseeding with the most competitive of adapted (native or non-native) desired

species and careful post-seeding grazing management to reduce the risk and

consequences of reinvasion. They may also require periodic treatment for

residual weeds. For whole landscapes dominated by noxious weeds, there may

be little option other than biological control. Yet biological controls are not

available for many weed species.

To accomplish vegetation management objectives suggested for this state

and transition model, care should be taken to avoid facilitating the spread of

invasive weeds. Expansions to the road network and soil disturbances increase

bare areas where invasive weeds can more easily establish. Virtually every

invasive weed population is first a roadside weed before its population explodes.

Many weed infestations begin in areas disturbed by machines, and some of

these are for vegetation management purposes. To prevent weed infestations

from spreading, it is important to routinely scout for new invasive weeds,

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especially in areas likely to be initial colonization areas (roadsides, waters,

riparian areas, turnout areas, corrals, utility corridors, borrow pits, treatment sites

etc.). Managers may also, remove or alter stresses on native perennial species

that can aid expansion from an affected area.


Description: Accelerated erosion has resulted in loss of topsoil, altered

hydrologic characteristics (i e. reduced infiltration and increased runoff), and

lowered water and nutrient storage capacity. These changes to the growing

environment have resulted in an altered ecological potential for the site. For

example, a Wyoming big sagebrush site may become a shadscale (Atriplex

confertifolia [Torr. & Frem.]), site. Lowered site potential means lowered

vegetation production, less soil protection, and increased soil loss until a new

equilibrium is reached.

Successional trajectories: The risk of transitioning to the altered site potential

state increases after transitioning to the shrub, tree, annual grass fire cycle or

perennial invasive weed states. The new site potential and the array of possible

plant species and successional trajectories greatly depend on the soil remaining

as the rate of soil erosion stabilizes. For very shallow soils, plants survive by

tolerating extended periods without available soil moisture or by sending roots

deep into rock fissures. Cheatgrass, a winter annual, survives drought as seeds

that do not germinate in some years and by developing seed early. On sites

where the topsoil has been eroded away, clayey subsoil often becomes exposed

at the surface. Roots must penetrate the heavy clay and tolerate any shrinking

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and swelling of the clayey soil during germination. Plants must then be able to

persist with less soil moisture than available within an intact, non-eroded soil.

Clayey sites are also susceptible to invasion by medusahead rye (Taeniatherum

caput-medusa L. Nevski.).

Management strategies: Because topsoil or even subsoil has been lost, return

of the native perennial herbs and shrubs characteristic of the perennial

herbaceous state depends on soil forming processes that are slow under most

conditions. The area should now be managed under the guidance provided by

the state and transition model and ecological site description for the new

ecological site if one is available that fits. It is expected that the species

composition and limited productivity of the vegetation established on the altered

site will have a low resiliency and minimal utility.

2) NATIVES ONLY

In these plant communities and landscapes, non-indigenous species are usually

not present. Only plant species native to the Great Basin are important in

ecological processes and management. If present, non-natives reflect a

disturbance of vegetation that has left an open niche that can be easily filled

through recovery of native vegetation. The general model (Figure 2.) may

describe historic ecological processes and is still relevant in some areas.

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sorbitol positive

Introduction

WYOMING BIG SAGEBRUSH STATE & TRANSITION MODEL

(NATIVES ONLY ON SITES SUPPORTING TREES)

SHRUB STATE TREE STATEPERENNIAL HERBACEOUS STATE

SUCCESSIONAL CONTINUUM

RESILIANT TWO-WAY MOVEMENT NON-RESILIANT,

FINANCIAL OR MANAGEMENT INVESTMENT REQUIRED TO MOVE VEGETATION BACK ACROSS A THRESHOLD

REL

ATI

VE

AB

UN

DA

NC

E

DININISHED HERBACEOUS UNDERSTORY

UNDERSTORY ABSENT

REQUIRESADDITIONAL INPUTS

REQUIRES ADDITIONAL INPUTS

DIMINISHED SHRUB

UNDERSTORY

?

EARLY SERAL DOMINANTS

HERBACEOUS

PERENNIALS

SAGEBRUSH (OVERSTORY)

DOMINANT TREE SPECIES (OVERSTORY)


UNKNOWN SPECIES COMPOSITION

SUCCESSION


NON-RESILIANT,

HERBACEOUS PLANT COMMUNITY

SHRUBBY OVERSTORYWITH HERBACEOUS

UNDERSTORY

UNDERSTORYABSENT

Figure 2. Each box is shown as a different color to identify that it is a different state. The solid-line arrows between boxes are irreversible transitions across thresholds. Dashed-line arrows show active restoration of ecological processes. Middle box shows relative abundance of plant groups and relative sequence of transitions through succession without proactive management.

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PERENNIAL HERBACEOUS STATE

Description: The plant community is dominated by deep-rooted perennial

bunchgrasses, perennial forbs, and varying amounts of Wyoming big sagebrush.

Sagebrush can dominate the plant community and juniper and/or pinyon pine

trees may be present as seedlings, saplings, or very sparse mature trees, as

long as the understory remains robust. If the perennial understory is dense and

vigorous enough to recover after being released from the competition of woody

plants, the vegetation has not crossed a threshold to the shrub or tree state.

Descriptions of the ecological sites listed in Table 1 provide relative species

composition and production data for each ecological site in this perennial

herbaceous state.

Successional trajectories: Plant community is resilient or cyclic because

secondary succession processes and disturbance regimes are functional. Life-

form dominance (species composition) is controlled primarily by fire, although

Aroga moth or other phenomena can also thin or kill patches of Wyoming big

sagebrush. Normal fire frequency is about 50-100 years (Wright and Bailey

1982). Without periodic woody plant removal, the plant community will transition

toward the shrub state or if trees are adjacent to the site, to the tree state. On

drier sites juniper may increase and on more mesic sites, pinyon may increase.

Following wildfire, sprouting shrubs may dominate but will be gradually replaced

by perennial bunchgrasses and sagebrush. If the area is devoid of big

sagebrush, it could be restored through time with seeds from surrounding areas

and could be re-established more quickly with seeding. Poor grazing

management of large domestic and/or wild herbivores can diminish the vigor and

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expression of deep-rooted perennial herbaceous plants leaving primarily

Sandberg bluegrass. This makes soil moisture and other site resources more

available to competitive shrubs and/or trees and accelerates and increases the

likelihood of the transition to the shrub or tree state. If the perennial understory is

too sparse or weak to recover quickly after being released by fire or other major

disturbance, the vegetation has crossed a threshold to the shrub or tree state.

Large fires that remove sagebrush leaving no islands and/or repeated fires that

remove succeeding generations before reaching reproductive age (about 5 yrs)

may create large landscape areas with few or no sagebrush plants for extended

periods.

Management strategies: To maintain the perennial herbaceous state, limit

development of shrub or tree cover to what is appropriate for a resilient

herbaceous understory on the site. Research indicates that herbaceous biomass

production begins to decline when sagebrush cover reaches 5-7% and density

begins to decline when sagebrush cover reaches about 12-15%, depending upon

site potential (Rasmussen and others 2001, Winward 1991). Intervene with

prescribed fire, herbicide, mechanical control measures, and/or prescription

grazing. Grazing can be managed to reduce stress to palatable species,

especially during the growing season, slowing the advance of woody species.

Grazing can also be used to accelerate the process of sagebrush recolonization

after a fire. Shrub decrease can be fostered by relatively intense winter grazing

by animals with shrub diet preferences. Concentration of livestock at feeding

sites can reduce shrub density through mechanical damage to sagebrush,

especially when shrubs are frozen or dry and brittle. Grazing prescriptions

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should strive to maintain the vigor of the herbaceous community. Where the

perennial herbaceous understory is weak and shrub cover is still well below

maximum, investigate the feasibility of reseeding or adjusting grazing

management to improve vigor and density of desirable species in the understory

several growing seasons prior to controlling shrubs. Management to maintain

the perennial herbaceous state is often much more cost effective than

management to return to this state once a threshold has been crossed.

SHRUB STATE

Description: Herbaceous understory cover has decreased below a threshold

level. Shrub cover has increased above a threshold level. Wyoming big

sagebrush and/or sprouting shrubs dominate the plant community. Perennial

understory vegetation, especially deep-rooted bunchgrass, is incapable of

recovery after fire.

Successional trajectories: Native herbaceous understory declines

substantially from perennial herbaceous state levels and trends toward absence

when sagebrush cover approaches its maximum. If trees are present and not

controlled, the plant community will transition to a tree state. Because a

threshold has been crossed, transition to the seeded perennial herbaceous state

requires fire or other shrub control measures, reseeding operations and follow-up

management. Removal of grazing pressure alone will not restore the native

herbaceous understory characteristic of the perennial herbaceous state or

reduce shrub abundance. Burning or other shrub or tree control measures alone

will not return the mix of deep-rooted bunchgrasses and other perennial

herbaceous plants because the seed bank and seed source have been depleted.

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Woody plant removal will release fire-adapted shrubs and create open areas for

early-seral species and increase erosion potentials.

Management strategies: To maintain sagebrush stands, prevent wildfire but

control junipers and/or pinyon pines as needed. To transition to the seeded

perennial herbaceous state, apply shrub control measures in conjunction with

reseeding. Shrub control measures could include prescribed fire, herbicide,

mechanical, or grazing. Because one or more thresholds have been crossed,

reseeding is essential after wildfire. Reseeding with appropriate seedbed

preparation, planting date, and other methods should include a mix of adapted

desired (native or non-native) grass, forb, and shrub species. Where perennial

herbaceous understory is missing and shrub cover is still well below maximum,

investigate the feasibility of reseeding several growing seasons prior to

controlling shrubs. The goal of re-establishing a desired herbaceous component

may require a multi-step approach through many years or decades.

TREE STATE

Description: Juniper and/or pinyon pine has established on the area due to

extended fire return interval. Although trees generally establish under shrub

canopies, they can invade both the perennial herbaceous and shrub states. The

understory (herbaceous and shrub) has decreased below a threshold level

because tree cover has increased above a threshold level. Trees dominate the

plant community, with leaf biomass and fuel buildup often to seven to eleven

times the level of the perennial herbaceous or shrub states. However, canopy

cover is concentrated, often leaving large bare interspaces with erosion rills. The

perennial understory can no longer respond to fire or other tree-removing

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disturbances because seed banks, seed sources, and antecedent plants have

been depleted.

Successional trajectories: The perennial herbaceous and/or shrub understory

declines from previous state levels to almost absent as trees attain mature height

at normal density. Shrub leaf biomass declines to about 20-25% of potential

when tree leaf biomass approaches 50% of maximum potential for the site

(Tausch and West 1995). Each 1% increase in tree cover can lead to about a

5% decline in understory production. Post fire vegetation is dominated by early

seral species and the limited number of species that had survived tree

dominance and fire or other tree removal. Because one or more thresholds have

been crossed, transition to the seeded perennial herbaceous state requires

reseeding and tree control. If native perennial understory is absent, fire or other

tree control measures alone will not increase most herbaceous/shrub understory

species to levels found in the perennial herbaceous state. Species from the

surrounding landscape with wind-blown seeds will be favored. Removing trees

will create open areas susceptible to invasive species, sagebrush, and fire-

tolerant shrubs and/or accelerated erosion. After fire, major soil erosion from

severe wind or major precipitation events on moderate or steeper slopes can

trigger a transition to an altered site potential state. Accelerated soil erosion can

also occur in large bare interspaces where rills can develop. This is more

common on arid sites.

Management strategies: To manage this state for continued tree production,

protection from wildfire is essential. However, as trees grow, fuel accumulates

and tree canopies grow closer to each other increasing the likelihood of a hot

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crown fire spreading across the landscape. Thinning to reduce crown cover, fuel

load, and fuel continuity is critical to long-term maintenance of a woodland plant

community. Continued net fuel production on this type often increases risk of fire

in neighboring woodland types, including areas where trees are much older

because fire was historically infrequent or lightening strikes caused only single-

tree fires. Management plans should be designed to break up the landscape

scale continuity of fuels with firebreaks, greenstrips, or imposed differences in

vegetation structure.

To transition to the seeded perennial herbaceous state, apply tree control

measures in conjunction with reseeding. Tree control measures could include

prescribed fire, herbicide, or mechanical treatments. Reseeding should include

adapted grass, forb, and shrub species and appropriate seedbed preparation,

planting date, and follow-up grazing management and weed control where

needed. Because one or more thresholds have been crossed, reseeding is

essential after wildfire.


Description: Accelerated erosion has thinned or eliminated topsoil, altered

hydrologic characteristics, and lowered water and nutrient holding capacity.

These changes alter the ecological potential of the site. Thereafter, reduced

vegetation cover and infiltration rate increases erosion potentials reducing site

potential until a new equilibrium is established.

Successional trajectories: New site potential, species composition of the

revegetation seed mix, and subsequent successional trajectories greatly depend

on characteristics of the remaining soil. For very shallow soils, plants survive by

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tolerating extended periods without available soil moisture or by sending roots

deep into rock fissures. On sites where clayey subsoil becomes exposed at the

surface after topsoil has eroded away, plants must tolerate any shrink-swell

characteristics of a clayey soil. Roots must be able to penetrate a heavy soil and

they must be able to persist with less available soil moisture.

Management strategies: Because topsoil or even subsoil has been lost, return

of native perennial herbs and shrubs characteristic of the perennial herbaceous

state depends on soil forming processes that are very slow under most

conditions. The area should now be managed under the guidance provided by

the state and transition model and ecological site description most similar to the

altered site. It is expected that the species composition and limited productivity

of the vegetation established on the altered site will have low resilience and

minimal utility.

CONCLUSION

This state and transition model and management key is designed to help

managers recognize opportunities to influence Wyoming big sagebrush sites in a

positive manner. It can be used for thinking at the site specific or the landscape

scale. Management opportunities are identified by determining the state and

successional trajectory by examining the vegetation. Pathways toward

thresholds indicate a need for action to prevent a transition to an unwanted state.

Thus, the model and management key helps set short-term and long-term

management objectives. Usually these objectives call for restoring resilience to

the perennial herbaceous state, protecting woody states from fire, focusing weed

control on prevention of high impact invasive weed population explosions, and

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reseeding immediately after a shrub or tree state burns. Management actions

are less risky, less expensive, and more satisfying when and where important

biological diversity remains and before difficult species, dangerous fuels, or

accelerated soil erosion dominate ecological processes. That is, before crossing

a threshold.

Across a landscape, the model helps focus attention on the highest priority

areas where an important management action or change has become urgent.

Across most landscapes, there are hot spots where site specific management is

urgently needed. There are other areas where the vegetation will remain resilient

into the future and areas where the threshold has been crossed. Crossing the

threshold to another state, decreases resilience and increases vegetation

treatment costs.

In general, the risk of losing the perennial herbaceous state is the highest

priority. This state cycles among a variety of species compositions to which

many wildlife and other species have adapted, including the many sagebrush-

dependent species. In addition many other resource values are produced in one

or more of the seral phases of this state. Its natural resistance to transitioning

across a threshold due to its resilience following natural disturbances makes this

state a low-cost management objective. However, after the introduction of exotic

invasive weeds and a century of altered fire regimes, this state is often at risk

where it still remains. Its increasing scarcity, and the presence of invasive weeds

that can more easily dominate after transitioning to the shrub or tree state,

elevates its value and its priority for management. Where it no longer remains,

the seeded perennial herbaceous state is its closest alternative.

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However, management does not equal preservation without disturbance.

This state is maintained by periodic disturbance. Between fires, time, succession

and growing season herbivory on the understory favors shrubs and trees. Fires

are the way nature balances these stresses to favor the understory. The focus of

land management in the Wyoming sagebrush type is to use management tools to

simulate natural disturbances at the right times and with the right combination of

other actions.

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REFERENCES:

Bestelmeyer, Brandon T., Joel R. Brown, Kris M. Havstad, Robert Alexander,

George Chavez, and Jeffrey E. Herrick. 2003. Development and use of

state-and-transition models for rangelands. J. Range Management,

56(2):114-126.

Laycock, W. A. 1991. Stable states and thresholds of range condition on North

American rangelands. J. Range Management 44(5): 427-433.

Rasmussen, G.A., M.P. O’Neill, and L. Schmidt. 2001. Monitoring Rangelands:

Interpreting What You See. Utah State Cooperative Extension Pub. NR

503.40p.

Stringham, Tamzen K., William C. Krueger, and Patrick L. Shaver. 2003. State

and transition modeling: An ecological process approach. J. Range

Management, 56(2):106-113.

Tausch, Robin J. and Neil E. West. 1995. Plant species composition patterns

with differences in tree dominance on a southwestern Utah pinon-juniper

site. Pages 16-23 In: Shaw, Douglas W., Earl F. Aldon, and Carol LoSapio

tech. Coord. Desired Future Conditions for Pinon-Juniper Ecosystems;

Proc. Of the Symp. 1994 Aug. 8-12 Flagstaff, Az USDA FS Gen Tech

Rept. RM-258.

Winward, A.H. 1991. A renewed commitment to management of sagebrush

grasslands. P. 2-7. In: Management of the sagebrush steppe. Agri. Exp.

Stat. Spec. Rept. 880. Oregon State University, Corvallis, OR.

the mission of the nevada agricultural experiment …naes.agnt.unr.edu/pms/pubs/50_2009_01.pdfthe...

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